EGCA,
As I recall, brain cells fading and all that, Sheffield City Airport was the project of a pilot, a BCal pilot if I recall correctly.
The DHC7 was a recent thing in those days, PLH became developed for the DHC7, LCY was developed for the DHC7 and when LCY first opened it was a purely DHC7 airport with operations by Brymon and the Airlines of Britain Group (British Midland, Loganair, Manx etc) jumped on the DHC7 bandwagon with their 'London City Airways' and Sheffield City Airport was another "Jump on the DHC7 bandwagon'.
Alas the DHC7 was slow (circa 215kts) and, in less than ideal weather conditions, uncomfortable and, with 4 engines, more expensive to operate, I recall a route or few were considered from Sheffield, they didn't work out and the rest is history.
With regards to Staverton it's too close to, particularly, BHX for regular scheduled services besides the Channel Islands or similar, BRS and CWL also surround Staverton and LHR is only down the A40/M40.
As you point out CVT is crying out for work and CVT has an Airbus/Boeing sized runway, they're doing similar at Kidlington as they're doing at Staverton, and not forgetting West Wales Airport being developed at Aberporth, what is it about aviation that turns knowledgable and experienced businessmen who should know better in to 'boys and their toys'?
There simply isn't enough revenue passenger work around to support all these 'wannabee an airport' airfields!